(1) Field of Invention
This invention relates to the synthesis of crystalline, combined porous inorganic and organic oxide materials possessing uniform framework-confined mesopores in the range 1 to 102 nm. The organic oxide precursors are hydrolyzable organic silanes which are incorporated into the framework. In particular, the present invention relates to such materials where the formation of the mesoporous structure is accomplished by a self-assembly mechanism involving complexation and/or hydrogen (H) bonding between aqueous or alcoholic emulsions of various nonionic poly(oxyalkylene) based surfactants and various mixed neutral inorganic and organic oxide precursors. This is followed by hydrolysis and subsequent condensation of hydrolysis products at ambient reaction temperatures. This templating approach allows for the removal of template through solvent extraction.
(2) Description of Prior Art
Modern human activities rely greatly upon porous solids of both natural and synthetic design. The pore structures of such solids are generally formed during crystallization or during subsequent treatments. These solid materials are classified depending upon their predominant pore sizes: (i) microporous, with pore sizes  less than 1.0 nm; (ii) macroporous, with pore sizes exceeding 50.0 nm; and mesoporous, with pore sizes intermediate between 1.0 and 50.0 nm. Macroporous solids find limited use as adsorbents or catalysts owing to their low surface areas and large non-uniform pores. Micro- and mesoporous solids however, are widely utilized in adsorption, separation technologies and catalysis. There is an ever increasing demand for new, highly stable well defined mesoporous materials because of the need for ever higher accessible surface areas and pore volumes in order that various chemical processes may be made more efficient or indeed, accomplished at all.
Porous materials may be structurally amorphous, para-crystalline or crystalline. Amorphous materials, such as silica gel or alumina gel, do not possess long range crystallographic order, whereas para-crystalline solids such as xcex3- or xcex7-alumina are semi-ordered, producing broad X-ray diffraction peaks. Both these classes of materials exhibit very broad pore distributions predominantly in the mesoporous range. This wide pore distribution however, limits the effectiveness of catalysts, adsorbents and ion-exchange systems prepared from such materials.
Zeolites and some related molecular sieves, such as alumino-phosphates and pillar interlayered clays, possess rigorously uniform pore sizes. Zeolites are highly crystalline microporous aluminosilicates where the lattice of the material is composed of IO4 tetrahedra (I=Al, Si) linked by sharing the apical oxygen atoms. Cavities and connecting channels of uniform size form the pore structures which are confined within the specially oriented IO4 tetrahedra (Breck, D. W., Zeolite Molecular Sieves: Structure, Chemistry and Use; Wiley and Sons; London, pages 1 to 100 (1974)). Zeolites are considered as a subclass of molecular sieves owing to their ability to discriminate small molecules and perform chemistry upon them. Molecular sieves in general are materials with crystalline frameworks in which tetrahedral Si and/or Al atoms of a zeolite or zeolitic lattice are entirely or in part substituted by other atoms such as B, Ga, Ge, Ti, Zr, V, Fe or P. Negative charge is created in the zeolite framework by the isomorphous substitution of Si4+ ions by Al3+ or similar ions. In natural zeolites, this charge is balanced by the incorporation of exchangeable alkali or alkaline earth cations such as Na+, K+, Ca2+. Synthetic zeolites utilize these and other cations such as quaternary ammonium cations and protons as charge balancing ions. Zeolites and molecular sieves are generally prepared from aluminosilicate or phosphate gels under hydrothermal reaction conditions. Their crystallization, according to the hereafter discussed prior art, is accomplished through prolonged reaction in an autoclave for 1-50 days and oftentimes, in the presence of structure directing agents (templates). The correct selection of template is of paramount importance to the preparation of a desired framework and pore network. A wide variety of organic molecules or assemblies of organic molecules with one or more functional groups are known in the prior art to provide more than 85 different molecular sieve framework structures. (Meier et al., Atlas of Zeolite Structure types, Butterworth, London, pages 451 to 469 (1992)).
Recent reviews on the use of templates and the corresponding structures produced, as well as the mechanisms of structure direction have been produced by Barrer et al., Zeolites, Vol. 1, 130-140, (1981); Lok et al., Zeolites, Vol. 3, 282-291, (1983); Davis et al., Chem Mater., Vol. 4, 756-768, (1992) and Gies et al. Zeolites, Vol 12, 42-49, (1992). For example, U.S. Pat. No. 3,702,886 teaches that an aluminosilicate gel (with high Si/Al ratio) crystallized in the presence of Quaternary tetrapropyl ammonium hydroxide template to produce zeolite ZSM-5. Other publications teach the use of different organic templating agents and include; U.S. Pat. No. 3,709,979, wherein quaternary cations such as tetrabutyl ammonium or tetrabutyl phosphonium ions crystallize ZSM-11 and U.S. Pat. No. 4,391,785 demonstrates the preparation of ZSM-12 in the presence of tetraethyl ammonium cations. Other prior art teaches that primary amines such as propylamine and i-propylamine (U.S. Pat. No. 4,151,189), and diamines such as diaminopentane, diaminohexane and diaminododecane (U.S. Pat. No. 4,108,881) also direct the synthesis of ZSM-5 type structure. Hearmon et al (Zeolites, Vol. 10, 608 -611, (1990)) however, point out that the protonated form of the template molecule is most likely responsible for the framework assembly.
Thus the synthesis and characterization of mesoporous molecular sieves, high surface area metal oxides (800 to 1400 m2gxe2x88x921) with uniform pore sizes (20 to 100 xc3x85 in diameter), have in recent years commanded much attention in the field of materials chemistry (Kresge, C. T., et al., 359, 710 (1992); Monnier, A., et al., Science, 261, 1299 (1993); Tanev, P. T., et al., Science 267, 865 (1995); Tanev, P. T., et al., Chem. Mater. 8, 2068 (1996); Bagshaw, S. A., et al., Science, 269, 1242 (1995); and Prouzet, E., et al., Angew. Chem. Int. Ed. Eng. 36, 516 (1997). These materials can be prepared by the assembly and subsequent co-condensation of metal oxide precursor molecules (such as TEOS, Si (OEt)4) around structure directing micelles consisting of surfactant molecules which can be either charged (such as alkyltrimethylammonium ions (Kresge, C. T., et al., Nature, 359, 710 (1992); and Monnier, A., et al., Science, 261, 1299 (1993)) or electrically neutral (such as primary alkylamines (Prouzet, E., et al., Angew. Chem. Int. Ed. Eng. 36, 516 (1997); Brunel, D., et al., Stud. Surf. Sci. Catal. 97, 173 (1995); and U.S. Patent No. 5,622,684 to Pinnavaia et al).
Focus has recently been put on researching methods to functionalize these materials in order to make mesostructured oxides useful for chemical applications. The chemical modification of mesoporous molecular sieves was first achieved by the incorporation or grafting of suitable moieties onto the surface of a preformed mesostructured oxide (Brunel, D., et al., Stud. Surf. Sci. Catal. 97, 173 (1995); and Cauvel, A., et al., AIP Conf. Proc. 354-477 (1996)), producing highly effective adsorbents (Mercier, L., et al., Adv. Mater. 9, 500 (1997); and Feng, X., et al., Science 276, 923 (1997)) and catalysts (Tanev, P. T., et al., Nature 368, 321 (1994); and Maschmeyer, T., et al., Nature 378, 159 (1995)). U.S. Patent Nos. 5,446,182 and 5,318,846 to Bruening et al describe various silane compounds with a liquid covalently bonded through an organic space to a solid support. The silane compounds disclosed in these patents can be starting materials for the compounds of the present invention and they are incorporated by reference herein. All of these compounds are xe2x80x9chydrolyzablexe2x80x9d organic silicon alkoxides, commonly referred to as xe2x80x9csilanesxe2x80x9d. The patents describe them in reactions with preformed solid substrates (silica gel, for instance). In this instance the silicon compound is grafted to the surface of the substrate rather than an integral part of the matrix. An alternate synthetic strategy was subsequently reported in which functional organic groups were directly incorporated into the mesostructures by a one-step process involving the co-condensation of tetraethoxysilane (TEOS) and organosilane (Si(OEt)3R, where R is a functionalized organic group) in the presence of structure-directing alkyltrimethylammonium surfactant micelles (Burkett, S. L., et al., J. Chem. Soc. Chem. Commun. 1367 (1996); Fowler, C. E., et al., J. Chem. Soc. Chem. Commun. 1769 (1997); Lim, M. H., et al., Chem. Mater. 10, 467 (1998); Van Rhijn, W. M., et al., J. Chem. Soc., Chem. Commun. 317 (1998)). This technique appears to be a convenient improvement over post-synthesis functionalization because the preparation of ordered porous materials with controlled chemical composition can be achieved by adjusting the stoichiometry of the synthesis mixture. Since charge-bearing ammonium surfactants are used as structure-directing agents in this technique, an acid leaching technique was required to remove the surfactant (by ion-exchange) from the electrically charged frameworks, the result of which sometimes leading to the structural decomposition of the materials (Burkett, S. L., et al., J. Chem. Soc. Chem. Commun. 1367 (1996)). By using a non-electrostatic assembly strategy to prepare such functionalized mesostructures, the resulting frameworks would be electrostatically neutral, allowing the removal of the structure-directing surfactant by simple solvent extraction. Such an approach was demonstrated by Macquarrie for the preparation of ordered hybrid organic-inorganic silicas, in which a charge-neutral surfactant (dodecylamine) was used to assemble the mesostructures, then extracted out of the pore channels using hot ethanol (Macquarrie, D. J., J. Chem. Soc., Chem. Commun. 1961-1962 (1996)).
There is a need for improvements in the preparation of organic-inorganic metal oxide compositions which in particular are active for metal binding. The prior art structures are difficult to synthesize or have pores which are especially useful for metal binding.
It is therefore an object of the present invention to provide a new approach to the design and synthesis of mixed porous inorganic and organic metal oxide compositions with well defined mesoporosity, and controlled elementary particle size. Further, it is an object of the present invention to provide inexpensive templates, precursors and methods. Further, it is an object of the present invention to provide a template system that allows for facile recovery and thereby recycling of the template from the condensed organic-inorganic structure via solvent extraction. Further, it is an object of the present invention to provide a template system that affords mesoporous materials through lower cost, lower toxicity than either quaternary ammonium or amine surfactants and template biodegradability. It is a further object of the present invention to provide for the preparation of functionalized non-layered mesoporous structures of inorganic-organic oxide materials. Yet another objective is to provide for non-layered mesoporous structures of inorganic oxide materials derived from metals other than silicon, that are not accessible through the prior art. These and other objects will become increasingly apparent by reference to the following description and the drawings.
The present invention relates to a synthetic, semi-crystalline inorganic oxide composition prepared by a method comprising reacting an inorganic oxide precursor and a hydrolyzable organic silane in the presence of water and a structure-directing template which comprises a poly(oxyalkylene) based surfactant having a hydrophobic and hydrophilic moiety, the composition having at least one resolved x-ray reflection corresponding to a lattice spacing of 3 to 40 nm, framework confined pores between about 1 and 35 nm, and a surface area between 100 and 1500 m2/gm and with the hydrolyzed organic silane moiety incorporated into the framework pore walls. The organic functional groups of the inorganic-organic oxide materials allow their use as adsorbents for the removal of organics from water, as trapping agents for the removal of toxic metal ions from water, as immobilized acids or bases and as heterogeneous catalysts for a wide range of organic reactions.
The present invention also relates to a method for the preparation of a synthetic, semi-crystalline hybrid organic-inorganic silicon oxide composition which comprises:
(a) providing a mixture of (i) a silicon oxide precursor; (ii) a neutral polymeric surfactant which comprises poly(oxyalkylene) polymer as a template having a hydrophobic and a hydrophilic moiety; and (iii) a hydrolyzable organic alkoxy silane;
(b) mixing the solution to form a precipitated composition in the presence of a solvent comprising water; and
(c) removing the surfactant from the precipitated composition by solvent extraction to form the organic-inorganic silicon oxide composition.
The silane compound used in preparing the compositions has the formula:
X4xe2x88x92nSiRn
where n=1, 2 or 3. Compounds where X is 1 are preferred. X can be an alkoxide or a halide.
The anhydrous oxide is represented by the formula
SiO2(SiO1.0R2.0)x
where SiO1.0R2.0 is a hydrolyzed organic silane moiety incorporated into the framework, Si is a silicon atom, O is oxygen and x is between 0.01 and 0.25.
The initial inorganic oxide precursor can be an alkoxide, a halide or a 1,3-diketonate. Preferred is the alkoxide. The hydrolyzable organic silane provides the hydrolyzed organic silane, organic groups which are a sulfur containing ligand moiety.